Method of preparing magnetic garnet crystals



United States Patent 3,051,656 METHOD OF PREPARING MAGNETIC GARNETCRYSTALS Bernhard Krarnarsky, Microwave Chem. Lab. Inc. 282 7th Ave.,New York, N.Y. No Drawing. Filed May 8, 1959, Ser. No. 811,807 7 Claims.(Cl. 252-625) This invention is concerned with methods of growingmagnetic garnet crystals.

More specifically, this invention has to do with a significantimprovement in method whereby single crystals of certain rare-earth irongarnets may be produced which are considerably larger in average sizeand in far greater yield.

The identity of these ferrimagnetic garnets, whose general formula is MFe O (M=rare earth, e.g. yttrium, gadolinium, erbium, Samarium) wasfirst reported in 1956. (F. Bertaut and F. Forrat: Comptes Rendus 242,382, 1956; S. Geller and M. A. Gilleo; Acta Cryst. 10, 239, 1957.) Thefirst efforts to prepare these compounds resulted in polycrystallineproducts. Interest in the physical properties of these garnets and theirpotential use in microwave systems indicated the desirability of growingsingle crystals of these compounds of maximum size. At present, singlemagnetic garnet crystals have found important use in parametricamplifiers for radar systems; and because they have the property oftransparency to infra-red, their use as infra-red transmissionmodulators is projected. I. P. Rerneika (J. Amer. Chem. Soc. 78, 4259,1956) first reported single crystals. J. W. Nielsen and E. F. Dearborn(J. Phys. Chem. Solids 5, 202-207, 1958) have investigated and outlinedthe general methods of producing these magnetic garnets as singlecrystals from three-component systems, i.e. iron oxide, Fe O with leadoxide, PbO, as flux.

Nielsen has defined the composition range in which magnetic garnets willgrow and the conditions favorable to their growth. However, his methodshave produced at best crystals of generally small size and poor yield,with only an occasional large crystal.

The present invention departs from Nielsens method by adding a fourthcomponent to the system, which suppresses the formation of many nucleifor multiple crystal growth, and encourages the formation of few, largerin dividual crystals. It has been found that the addition of suitablequantities of bon'c anhydride, B to the threecomponent system of Nielsenprovides a remarkable increase in the size and yield of magnetic garnetcrystals.

The following table of typical experimental results clearly shows theeifect of the addition of boric anhydride:

Percent Average Total Expt Composition B203 Yield of Size Wt. of No, ofBulk Added Yttrium- (Diarn) Y,I. G. Material (In iron of Y.I.G. Single(In Mole Mole Garnet Single Crystals Percent) Percent) Single Crystals(Grams) Crystals PbO:52.5 1 FezOaz44 0 6. 04 0. 09 1. 92

YzO325.25--

1 Better crystal faces than in 1.

lQQ

The typical procedure employed in the above experimental batchescomprised the following steps: A thorough mixture of the oxide powdersin the proportion indicated was made, and a weighed portion thereofplaced in a 375 ml. platinum crucible so as to fill it. (The averagebatch weight was 400 gms.) The crucible was covered tightly with aplatinum lid and placed in a furnace. The furnace temperature was raisedto 1325 and maintained at that point for 10 hours to insure completesolution of the ingredients. The temperature was then lowered at asteady rate of l.9 per hour, to allow for optimum crystal growth, until925 C. was reached.

The crucible and its contents were then cooled rapidly to roomtemperature, and the resultant mass repeatedly boiled in dilute nitricacid and washed until the matrix was sufficiently reduced to permitdislodgement of the crystals within. Separation of the garnet from thenon-garnet crystals was accomplished magnetically in the usual manner,taking advantage of the difference in Curie temperatures of theaforesaid crystals.

The magnetic garnet crystals formed in the above manner arewell-developed in crystal structure and quite pure, the total impuritiesrunning less than 0.2%, and the quantity of boric anhydride in thecrystals less than 0.09% determined spectroscopically. The weight ofthese crystals ranges from mg. to 2.0 gms. when prepared as above. Itshould be noted here that while the crystal sizes and yields in theseexperimental batches are comparable among themselves, better yields andlarger crystals are to be expected from using larger crucibles andbigger batches.

Returning to consideration of the experimental results shown in thetable above, it is evident that the range of effectiveness of the B 0additive is clearly marked and quite specific. A significant influenceis shown when 5 mole percent is added; the optimum amount appears to bein the range of 10 mole percent; and over 15 mole percent, no garnetcrystals form, since new phases appear and glassy products result.

The function of the boric anhydride additive is evidently that ofnucleus suppression, similar to that of mineralizers, so that a fewlarge crystals, rather than many small crystals, are encouraged to grow.Variation of the other three components, namely, lead oxide, iron oxide,and rare-earth oxide will influence the formation of garnet crystals asindicated in the phase diagrams of Nielsen, as will modifications oftemperature, batch size, etc. However, as indicated in Experiment 6 inthe table above, under conditions Where garnet crystals Will form at allin the three-component system, the addition of boric anhydride in theindicated range will enhance the yield and quality of said crystals.

Suitable ranges within the contemplation of the invention are asfollows:

Iron oxide, F6 0,, from 25 to 70 mole percent Lead oxide, PbO, from 30to 70 mole percent Rare-earth oxide, M 0 from 0.1 to 10 mole percentBoric anhydride, B 0 from 2 to 15 mole percent The letter M in theforegoing is intended to represent a rare-earth selected from the groupcomprising yttrium, gadolinimum, erbium and samarium.

The foregoing mixtures in the ranges given may be heated to atemperature in the range of 1315 to 1345 degrees centrigrade. The mosteffective temperature, however, is 1325 degrees centigrade. It will beunderstood that the material should be maintained at said elevatedtemperature for a minimum of 8 to 10 hours under standard pressure. Thecooling rate ranges from 0.5 degrees centigrade per hour, the mosteifective rate being 1.9 degrees per hour. Cooling should continue untiloptimum or maximum crystal growth is attained. It will be found thatthis will occur by the time the temperature is reduced to approximately925 degrees.

I claim:

1. The method of growing magnetic iron rare-earth garnet crystals,comprising the steps of providing a mixture of iron oxide, lead oxide,rare-earth oxide and boric anhydride from 2 to 15 mole percent, heatingsaid mixture to an elevated temperature suflicient to cause it to gointo solution, and then cooling said heated mixture at a controlled,sufficiently slow rate to produce crystals.

2. The method of growing magnetic iron rare-earth garnet crystals,comprising the steps of providing a mixture of iron oxide, from 25 to 70mole percent, lead oxide, from 30 to 70 mole percent, rare-earth oxide,from 0.1 to 10 mole percent, the rare earth being selected from thegroup consisting of yttrium, gadolinium, erbium and Samarium, and boricanhydride, from 2 to 15 mole percent, heating said mixture to atemperature in the range of 1315 to 1345 degrees centigrade, and thencooling said heated mixture at a controlled rate in the range of 0.5 to5 degrees centigrade per hour to allow for proper crystal growth.

3. The method of growing magnetic iron rare-earth garnet crystals,comprising the steps of providing a mixture of iron oxide, from 25 to 70mole percent, lead oxide, from 30 to 70 mole percent, yttrium oxide,from 0.1 to mole percent, and boric anhydride, from 2 to mole percent,heating said mixture to a temperature in the range of 1315 to 1345degrees centigrade, and then cooling said heated mixture at a controlledrate in the range of 0.5 to 5 degrees centigrade per hour to allow forproper crystal growth.

4. The method of growing magnetic iron rare-earth garnet crystals,comprising the steps of providing a mixture of iron oxide, approximately44 mole percent, lead oxide, approximately 52.5 mole percent, yttriumoxide, approximately 3.5 mole percent, and boric anhydride,approximately 10.34 mole percent, heating said mixture to a temperaturein the range of 1315 to 1345 degrees centigrade, and then cooling saidheated mixture at a controlled rate in the range of 0.5 to 5 degreescentigrade per hour to allow for proper crystal growth.

5. The method of growing magnetic iron rare-earth garnet crystals,comprising the steps of providing a mixture of iron oxide, approximately43.75 mole percent, lead oxide, approximately 51.5 mole percent, yttriumoxide, approximately 5.25 mole percent, and boric anhydride,approximately 10.34 mole percent, heating said mixture to a temperaturein the range of 1315 to 1345 degrees centigrade, and then cooling saidheated mixture at a controlled rate in the range of 0.5 to 5 degreescentigrade per hour to allow for proper crystal growth.

6. The method of growing magnetic iron rare-earth garnet crystals,comprising the steps of providing a mixture of iron oxide, approximately44 mole percent, lead oxide, approximately 52.5 mole percent, yttriumoxide, approximately 3.5 mole percent, and boric anhydride,approximately 10.34 mole percent, heating said mixture to a temperatureof approximately 1325 degrees centigrade, maintaining it at saidtemperature for approximately 10 hours, and then cooling said heatedmixture at the rate of 1.9 degrees centigrade per hour until atemperature of approximately 925 degrees centigrade is reached in orderto allow for proper crystal growth.

7. The method of growing magnetic iron rare-earth garnet crystals ofclaim 1, wherein the iron oxide is in a range from 25 to mole percent,the lead oxide ranging from 30 to 70 mole percent and the rare-earthoxide ranging from 0.1 to 10 mole percent.

References Cited in the file of this patent UNITED STATES PATENTS2,736,708 Crowley Feb. 28, 1956 2,848,310 Remeika Aug. 19, 19582,886,530 Greger May 12, 1959 2,938,183 Dillon May 24, 1960 2,957,827Nielsen Oct. 25, 1960 OTHER REFERENCES J. Amer. Ceram. Soc., Bauer etal., vol. 33, pp. -143.

Comptes Rendus, Aleonard et al., vol. 242, pp. 2531- 2533, May 23, 1956.

Comptes Rendus, Pauthenet et al., vol. 243, pp. 1499- 1501, Nov. 12,1956.

2. THE METHOD OF GROWING MAGNETIC IRON RARE-EARTH GARNET CRYSTALS,COMPRISING THE STEPS OF PROVIDING A MIXTURE OF IRON OXIDE, FROM 25 TO 70MOLE PERCENT, LEAD OXIDE, FROM 30 TO 70 MOLE, PERCENT, RARE-EARTH OXIDE,FROM 0.1 TO 10 MOLE PERCENT, THE RARE EARTH BEING SELECTED FROM THEGROUP CONSISTING OF YTTRIUM, GADOLINIUM, ERBIUM AND SAMARIUM, AND BORICANHYDRIDE, FROM 2 TO 15 MOLE PERCENT, HEATING SAID MIXTURE TO ATEMPERATURE IN THE RANGE OF 1315 TO 1345 DEGREES CENTIGRADE, AND THENCOOLING, SAID HEATED MIXTURE AT A COMTROLLED, RATE IN THE RANGE OF 0.5TO 5 DEGREES CENTIGRADE PER HOUR TO ALLOW FOR PROPER CRYSTAL GROWTH.